It depends on so much. The degree of numptiness, the offence, the circumstances. I used to holler at RLJers, I don't bother now, I just seethe inwardly, and shake my head for the benfit of onlookers. I've shouted "try getting some lights" to someone who came out of a side alley in the dark and almost hit me - although it wasn't really the lack of lights that was the issue, it was the not looking. I've shouted "try looking!" as well...
If someone appeared to have done something out of a general ignorance of the law or good practice (wearing a stupidly fitted helmet, or a dangerously dangling bag or something), I might offer advice. But I'd probably brace myself for a "what the F***s it got to do with you..."
If l "let rip" at every numpty, spanner, RLJer, non light using, helmet lite, pavement using, undertaking, swerving out without looking etc etc pedestrian on a bike. Where would i find the energy for letting rip at WVM, taxis, buses, BMW'ers, 4X4'ers, lorries etc who are all trying to kill me?
And the there are the pedestrians who appear from in between cars, from behind buses, randomly off the pavement, and especially those pushing prams, etc etc
I tried letting rip at everyone who deserved it in London once. Went hoarse and lost my voice after 500 yards.